RSF1, an Arabidopsis locus implicated in phytochrome A signaling

Abstract

In Arabidopsis, phytochrome A (phyA) is the major photoreceptor both for high irradiance responses to far-red light and broad spectrum very low fluence responses, but little is known of its signaling pathway(s). rsf1 was isolated as a recessive mutant with reduced sensitivity to far-red inhibition of hypocotyl elongation. At the seedling stage rsf1 mutants are affected, to various degrees, in all described phyA-mediated responses. However, in adult rsf1 plants, the photoperiodic flowering response is normal. The rsf1 mutant has wild-type levels of phyA suggesting that RSF1 is required for phyA signaling rather than phyA stability or biosynthesis. RSF1 thus appears to be a major phyA signaling component in seedlings, but not in adult, Arabidopsis plants.

Figure 1

rsf1 mutants are impaired in inhibition of hypocotyl elongation in FR and blue light. Data are means ± 2× se of at least 12 seedlings for each light treatment. All seedlings were grown at 22°C in continuous light. A, Col (black bar), rsf1 (white bar), and the appropriate photoreceptor mutants were grown for 6 d in 30 μmol m⁻² s⁻¹ blue (hy4, gray bar), 15 μmol m⁻² s⁻¹ R (phyB-9, hatched to the left bar), or 10 μmol m⁻² s⁻¹ FR (phyA-211, hatched to the right bar). B, Fluence rate response curve in continuous FR light of Col, rsf1, and phyA-211 seedlings. C, Fluence rate response curve in continuous R light of Col, rsf1, and phyB-9 seedlings.

Figure 2

Seedling phenotypes of rsf1 mutants. A, Wild-type, rsf1, and phyA-211 seedlings were grown for 4 d in continuous FR light 0.8 or 2 μmol m⁻² s⁻¹, representative seedlings were photographed. B, Picture of a representative wild-type and rsf1 seedlings grown for 4 d in continuous FR light on Suc-containing plates. Note that the mutant still accumulates anthocyanin. C, Quantification of anthocyanin accumulation in phyA-211, rsf1, and wild-type seedlings. Seedlings were grown on Suc-containing plates under 8 μmol m⁻² s⁻¹ FR light for 4 d. The experiment was done in triplicate with 10 seedlings for each measurement (mean ± 2× se).

Figure 3

rsf1 plants have no flowering time phenotype in LD or SD. LD are 16 of h light, 8 h of night; SD are 9 h of light, 15 h of night. Values are the means ± 2× se, with at least 18 plants for each photoperiod condition.

Figure 4

rsf1 mutants have wild-type levels of phyA and phyB. Proteins were extracted from 6-d-old seedlings, separated on 8% (w/v) SDS-PAGE gels, and blotted onto nitrocellulose. The membrane was probed with mAA1-3 or mBA2, monoclonal antibodies directed against phyA and phyB, respectively (Shinomura et al., 1996). The amido black-stained membrane is shown as a loading control. A, Seedlings were grown for 6 d in continuous 8 μmol m⁻² s⁻¹ FR light. B, Seedlings were grown for 6 d in the dark, or for the same total duration of which the last 1, 2, 4, or 15 h was in constant 100 μmol m⁻² s⁻¹ R light. C, Seedlings were grown for 6 d in the dark.